Since a magnesium alloy provides a weight saving effect because of its low specific gravity, it is widely used as an outer housing of a mobile phone or a portable audio equipment, a car component, a machine component, a structural material and the like. In order to further provide the weight saving effect, the magnesium alloy has to have both higher strength and ductility. In order to improve the above characteristics, the composition and component of the magnesium alloy is to be provided appropriately, and magnesium crystal grain constituting a matrix is to be miniaturized. Especially, regarding the miniaturization of the crystal grain of the magnesium alloy material, methods such as a rolling method, an extruding process method, a forging process method, a drawing process method, an ECAE (Equal Channel Angular Extrusion) method and the like have been used based on a plastic working process.
Japanese Unexamined Patent Publication No. 2001-294966 (patent document 1) discloses “magnesium alloy sheet, its manufacturing method and a product using the same”. According to the method disclosed in this document, a molten magnesium alloy is injected to mold a plate, and the plate is compressed and deformed by rolling and the plate is heat-treated to be re-crystallized to miniaturize a magnesium crystal grain.
Japanese Unexamined Patent Publication No. 2000-087199 (patent document 2) discloses “manufacturing method of rolled product of magnesium alloy, method of press working magnesium alloy, and press worked product”. According to the method disclosed in this document, a magnesium alloy plate is cold rolled at a predetermined reduction ratio of cross-sectional area and then the plate is heat-treated within a predetermined temperature range, so that the magnesium crystal grain is miniaturized due to re-crystallization.
According to methods disclosed in Japanese Unexamined Patent Publication No. 2001-294966 and Japanese Unexamined Patent Publication No. 2000-087199, an object to be processed is a plate material and the finally provided material is a plate material. Therefore, it is extremely difficult to manufacture a pipe material, a rod material and a material having a irregular configuration in section by the method disclosed in the above documents. In addition, it is necessary to perform a heat treatment after a rolling process, so that the cost of the material is increased.
Japanese Unexamined Patent Publication No. 2003-277899 (patent document 3) discloses “magnesium alloy member and its manufacturing method”. According to the method disclosed in this document, magnesium crystal grain is miniaturized by a first forging process, an aging heat treatment and a second forging process after a magnesium alloy material is solution heat treated. In this method also, since it is necessary to repeat the forging process and heat treatment several times, the cost for the material is increased. In addition, since it is essential that a predetermined process pre-strain is applied to the material in the first forging process, there is a limit in product configuration. Furthermore, the method disclosed in this document is not suitable for manufacturing a long size product such as a rod material or a pipe-shaped material.
International Publication WO03/027342A1 (patent document 4) discloses “magnesium-based complex material”. According to the method disclosed in this document, magnesium alloy powder or magnesium alloy chip is prepared as a starting raw material and this raw material is inputted in a mold mill and compression molding and extruding process are performed repeatedly to form a solidified billet of the powder or chip. Then, the hot plastic working is applied to the billet to provide high-strength magnesium alloy having a fine magnesium crystal grain. According to the method disclosed in this document, when a large solidified billet is manufactured, it is difficult to finely granulate the crystal grain uniformly. In addition, since it is necessary to considerably increase the number of processes of the compression and extrusion in order to make progress the fine granulation, the cost for the material becomes high.
Japanese Unexamined Patent Publication No.5-320715 (patent document 5) discloses “manufacturing method of magnesium alloy member”. According to the method disclosed in this document, the cuttings, scrap, waste product and the like discharged when the magnesium alloy member is cut are compressed and solidified and it is extruded or forged to manufacture a magnesium alloy member with a history of plastic working. At this time, the strength of the magnesium alloy is increased by urging the miniaturization of magnesium crystal grain by the plastic working.
In the above method, the crystal grain diameter of a magnesium matrix that determines the strength characteristics of the magnesium alloy after extruded or forged is strongly related not only to a strain amount applied to the raw material at the time of plastic working but also to the crystal grain diameter of the cuttings, scrap, waste product or the magnesium matrix of the forging material used as the starting raw materials. That is, the crystal miniaturization of the magnesium constituting the matrix of the starting raw material is extremely effective to increase the strength of the magnesium alloy material that is the final product. However, the crystal grain diameter of magnesium in the cutting, scrap, waste product or forging material used here is as huge as several hundreds micron. Therefore, it cannot implement considerable high strength and ductility in the magnesium alloy provided when the cuttings, scrap, waste material or forging material of the normal magnesium alloy are used as the starting raw material.
Meanwhile, focusing on a miniaturizing method of a magnesium crystal grain in a magnesium alloy powder that is one starting raw material, there is a rapidly quenching solidification process executed by a spray method or a single-roll method. According to the above method, while a molten magnesium alloy liquid drop is cooled and solidified for an extremely short time, the growth of the crystal grain is prevented, so that the magnesium-based alloy powder grain having fine crystal grain can be manufactured.
A cooling and solidifying rate depends on a cooled amount on the liquid drop surface. Namely, it depends on the specific surface area of the magnesium alloy liquid drop, so that the fine the liquid drop is, the higher the solidification rate and it can be solidified for a short time. As a result, the magnesium alloy powder has a fine crystal grain. Therefore, although the magnesium-based alloy powder having the fine crystal grain can be manufactured by the rapidly quenching method, since the crystal grain diameter becomes small on the other hand, the powder particle is likely to float during the manufacturing process, so that it is highly likely that dust explosion occurs. In addition, in a case of compression and solidification by die press molding, since fluidity is low in the fine powder particle, filling efficiency to the die is lowered and a space is partially formed and since the friction between the powder is increased, it is not likely to be solidified.
As described above, in order to implement the high ductility of the magnesium alloy, miniaturization of the magnesium crystal grain of the matrix is effective. In this case, first, a manufacturing method such as a forging method or die-casting method that does not go through a melting and solidifying process that involves the grain growth is required. More specifically, it is an issue to establish a solid phase process that molds and densely solidifies powder or a raw material having a geometric configuration similar to the powder within a temperature range below its melting point.
Next, it is necessary to make fine the crystal grain of the magnesium-based alloy powder used as a raw material at that time. At the same time, it preferably is relative coarse powder so as not to cause the dust explosion, and has an appropriate size in view of the press forming.